I took a class for making the rug. The key is to let the machine feed the strips, do not push or pull only guide the strips. The rounded ends are what determines the wave or cup formation. They have to be very relaxed and laying flat when sewing. Also not use a short or tight zig zag stitch. Some of the class had waves along the sides, one had a bowl effect. A few had very flat rugs. Mine was flat. I didn't press it while sewing. I was actually bored with it by the time I started sewing it together so just wanted it done by that time. In other words I didn't overly try getting it right.